Multisheet superplastically formed, diffusion bonded, metallic sandwich structures have been in use for many years, primarily in the aerospace industry, because of low cost, high temperature capability and good strength and stiffness per unit weight. Various processes for fabricating these structures have been developed in the past, with varying degrees of success, but all have proven slow to produce, and often they have high scrap rates. Parts produced by these prior art processes often are capable of only a fraction of the theoretical load-bearing capacity.
Most of the existing techniques for fabricating such structures, including the truss core technique shown in U.S. Pat. No. 3,927,817 to Hamilton, utilize superplastic forming of a stack of sheets in a die having a cavity shaped like the final sandwich structure. The stack includes two or more core sheets that are selectively joined to each other to form a core pack by lines of welding or diffusion bonding and top and bottom sheets that form the top and bottom outside skins of the sandwich structure. The stack is inflated at superplastic temperature with gas pressure to expand the top and bottom sheets outwardly against the interior walls of the die cavity to the desired exterior dimensions. During superplastic forming, the core sheets stretch away from their lines of attachment toward the top and bottom skins as those skins expand toward the boundary surfaces of the die cavity.
Early techniques for fabricating multi-sheet monolithic metal sandwich structures utilized diffusion bonding to join the core sheets along selective areas to produce the desired core structure. These techniques required accurate placement of stop-off to prevent diffusion bonding in areas where adjacent sheets were not intended to be bonded. Diffusion bonds retain superplastic qualities, but it has been difficult to produce a narrow, clean bond line that is free of stop-off. Diffusion bonding often is a lengthy process, requiring long holding times in the press at elevated temperature, preventing use of the press for other production. The capital intensive and time consuming nature of the diffusion bonding process lead to research into other techniques for joining the core sheets of multisheet stack that would be faster, more reliable, and less costly.
Another method, shown in U.S. Pat. Nos. 4,217,397 and 4,304,821 to Hayase et al., produces a metal sandwich structure having top and bottom face sheets and internal webs extending perpendicularly between the face sheets, defining closed cells within the sandwich structure. This method uses intermittent roll seam electric resistance welding of the core sheets along intersecting lines to establish the junction lines between the core sheets and to define the shape of the closed cells. The intermittent welding leaves gaps in the weld lines for passage of forming gas into the cells. This process was faster than the diffusion bonding technique, but still required care to avoid premature diffusion bonding of the core sheets to each other. The pack of sheets could be purged and pressurized to slightly inflate the stack and separate the sheets from one another so that they would not diffusion bond together. The pack of sheets would then be heated to superplastic temperature and forming gas would be admitted under pressure into the pack to expand the top and bottom sheets superplastically against the walls of the die cavity. Gas pressure was also admitted into the core pack to superplastically form the core sheets at the same time outward against the top and bottom sheets and to fold the core sheets over onto themselves about the weld lines to form the desired cellular sandwich structure. Diffusion bonding would occur where the core sheets contacted the face sheets or one another.
Heating titanium to a high temperature in the presence of oxygen creates a surface layer of alpha case, which is a hard but very brittle composition and is unacceptable in structural parts because of its tendency to crack. Such cracks could grow in a fatigue environment and lead to failure of the part. Consequently, it is desirable to purge oxygen and moisture from the stack of sheets before heating to elevated temperatures. In U.S. patent application Ser. No. 09/101,688 entitled "Multisheet Metal Sandwich Structure" by Buldhaupt et al., the stack of sheets is sealed and purged of oxygen and moisture before loading so the sealed pack can be loaded into a hot die without the danger of alpha case forming before the stack is purged and without using expensive press time to purge the stack and then slowly bring the die up to superplastic temperature.
Another technique for welding the sheets in the core pack together, shown in U.S. Pat. No. 4,603,089 to Bampton, uses a CO.sub.2 laser to weld sheets in the stack together. An improvement on the Bampton laser welding technique is shown in the Buldhaupt et al. patent application which teaches a practical way to hold the sheets together while they are being laser welded. It presses the sheets into intimate contact during welding to obtain a quality weld, and also protects the weld area from oxidation at high temperature that occurs during laser welding of titanium.
One solution for the problem of excessive thinout in superplastic forming a part having a central hole or opening is a double diaphragm forming technique. This technique achieves increased part thickness in the area of the part at the lip or periphery of the central hole or opening by using a blank having a hole in the area where the opening will be in the part. During forming, the hole in the blank increases in area while reducing stress in the material in the region, thereby reducing thinout in that region. A related disclosure is in U.S. Provisional Application No. 60/088,772 by Peter Smiley which uses slits in the runout area of the blank to reduce forming stresses in the material allowing the material to be drawn into the actual part region of the die, thereby minimizing thinout.
None of these prior techniques recognized the cause of a long-standing problem in the art, namely, the rupturing of the sheets of a core pack around the weld gap during forming of a metallic sandwich structure. When a new part is being developed, it is common for ruptures to occur in the core pack sheets in the region around the weld gap during superplastic forming because of excessive thinning. The forming gas can escape through these ruptures into the space between the core and face sheet, effectively terminating the forming process. The superplastic characteristics of the material in the heat affected zone around the weld is degraded compared to the material outside the heat affected zone, so it is difficult to optimize all the various process and material parameters for a given cell span and height by analysis during development. Such ruptures in the core prevent the part from forming properly, so it is immediately identified as a failed part and is scrapped. It is a source of increased development cost, increased weight when heavier gauge material must be used to prevent tears from occurring in the core pack sheets, and reduced production speed when longer forming times are required to prevent tearing. The problem has exasperated engineers and other workers in the art because the cause of the problem was not understood and because no reliable, consistent solution existed to correct the problem.